Treating the visual process as a casual chain, we use measurements of visual phenomena and visual performance to make inferences about how signals are transformed as they flow through the visual system. In particular we propose to exploit a nonlinearity recently demonstrated in the local visual response that precedes the convergence of signals from different photo- receptors, to analyse the characteristics and visual consequences of optical and neural processes that precede this nonlinearity and of the neural processes that come after it. It is possible to monitor the effects of this early local nonlinearity selectively (without intrusion by later nonlinearities, which are pervasive in the visual system), by stimulating the eye with grating patterns too fine to be resolved except at or near the receptoral level, where processing is still strictly local. When such patterns are briefly presented, keeping space-average luminance constant, the spatially modulated stimulus penetrates to and acts upon only those stages that can resolve the strips. But an early nonlinear process, transforming the signal at a stage where resolution is still preserved, can change the space-average excitation of later poorly resolving elements to an extent that depends upon the modulation of the unresolved grating. In many of our experiments we proceed by manipulating, more or less independently, the spatial and temporal characteristics of the stimulus gratings and of the distortion products derived from them. In others, we examine how the behavior of the nonlinear mechanism (as monitored through the visibility of its distortion products) is affected by the contrast of other stimulation. The stimuli used are interference fringes that are formed on the retinal at high contrast without substantial attenuation by the optics of the eye. Difference-frequency gratings are the main type of nonlinear distortion produce that we use for this purpose. Like moire' patterns they occur at a frequency equal to the vector difference in spatial frequency between the two grating stimuli that generate them. The role of nonlinear distortion in contrast sensitivity for a single grating is also to be investigated. These questions are approached both by threshold and by nulling methods.